ARES I-X LAUNCH VEHICLE MODAL TEST MEASUREMENTSAND DATA QUALITY ASSESSMENTS

J. D. Templeton, R. D. Buehrle and J. L. GasparNASA Langley Research Center

Hampton, VA

R. A. Parks and D. R. LazorNASA Marshall Space Flight Center

Huntsville, AL

ABSTRACT

The Ares I-X modal test program consisted of three modal tests conducted at the Vehicle AssemblyBuilding at NASA’s Kennedy Space Center. The first test was performed on the 71-foot 53,000-poundtop segment of the Ares I-X launch vehicle known as Super Stack 5 and the second test was performedon the 66-foot 146,000-pound middle segment known as Super Stack 1. For these tests, two 250 lb-peakelectro-dynamic shakers were used to excite bending and shell modes with the test articles resting on thefloor. The third modal test was performed on the 327-foot 1,800,000-pound Ares I-X launch vehiclemounted to the Mobile Launcher Platform. The excitation for this test consisted of four 1000+ lb-peakhydraulic shakers arranged to excite the vehicle’s cantilevered bending modes. Because the frequenciesof interest for these modal tests ranged from 0.02 to 30 Hz, high sensitivity capacitive accelerometerswere used. Excitation techniques included impact, burst random, pure random, and force controlled sinesweep. This paper provides the test details for the companion papers covering the Ares I-X finite elementmodel calibration process. Topics to be discussed include test setups, procedures, measurements, dataquality assessments, and consistency of modal parameter estimates.

INTRODUCTION

The Ares I-X launch vehicle is the first flight test vehicle for the Ares I launch vehicle, which isintended to be NASA’s replacement for the Space Shuttle. Ares I-X consists of a 4 segment solid rocketmotor with mass simulated hardware attached. The 4 segment solid rocket motor is the type that istypically used to assemble and launch the Space Shuttle. Because the Ares I vehicle will use a 5segment solid rocket motor, the lowest set of mass simulated hardware was a dummy 5 th segment.Above this hardware, the rest of the vehicle consisted of interstage and upper stage hardware that wasnot be recovered during the flight test. The upper stage consisted of a series of segments with internalplatforms to provide access up to the top of the launch vehicle. At the top of the launch vehicle was themass simulated hardware for the Crew Module and Launch Abort System (CM/LAS). The entire launchvehicle was assembled in the Vehicle Assembly Building of NASA Kennedy Space Center atop a MobileLauncher Platform, as shown in Figure 1. Ares I-X was successfully launched on October 28 th, 2009.

A series of three modal tests were performed on Ares I-X hardware from May through August 2009.These modal tests were intended to calibrate a finite element model so that the first three free-freebending mode pairs of the Ares I-X launch vehicle could be accurately predicted. The free-free modeswould then be compared to the modal parameters used in flight control system evaluations to ensure therobustness of the control system design. Based on the free-free modes of interest, the target test modeswere identified using pre-test analysis . The target test modes for the flight test vehicle on the MobileLauncher Platform were the first four bending mode pairs. In order to reduce the uncertainty in the finiteelement model before the flight test vehicle modal test, two component level modal tests were alsoproposed. The first of these modal tests was performed on the topmost hardware of the launch vehicleknown as Super Stack 5, and the second was performed on the 5 h segment and interstage hardware atthe middle of the launch vehicle known as Super Stack 1. This paper will focus primarily on the modaltest of the fully assembled Ares I-X launch vehicle, although all of the modal test setups will be discussed.

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TEST SETUPS

The modal test of the fully assembled Ares I-X launch vehicle mounted on the Mobile Launcher Platformwas known as the Flight Test Vehicle modal test. The instrumentation used for this modal test consistedof 90 PCB series 3701 capacitive accelerometers with 1 V/g sensitivity that were placed at 40 locationson the launch vehicle and Mobile Launcher Platform. Of these 40 acceleration measurement locations,19 were arranged in a triaxial configuration, 12 were biaxial, and 9 were uniaxial, as shown in Figure 2below. The instrumentation locations were selected based on pre-test analysis 1 so that the first fourbending mode pairs could be measured and distinguished from torsion modes and system modes withMobile Launcher Platform participation. In addition to the acceleration measurements, one strain gagebridge was measured at each of the four hold down posts where the launch vehicle was bolted to theMobile Launcher Platform. Additional strain gage bridge measurements at the hold down posts were alsoacquired during the modal test on a separate data acquisition system.

Figure 1. Fully assembled Ares I-Xlaunch vehicle2

Figure 2. Flight test vehicle modal test instrumentation locations

The primary method of excitation for the flight test vehicle modal test used four hydraulic shaker systems witha dynamic force capability of 1560 lb-peak and dynamic stroke of 2 in-peak. Due to safety concerns, the actualdynamic force capability available during the test was 560 lb-peak due to a lowered operating pressure. Thesesystems consisted of components manufactured by the Team Corporation. The hydraulic shaker actuators wereattached to a mounting frame that allowed the actuator to slide linearly on two hydraulic cylinders with adjustablestiffness as shown in Figure 3. Each shaker was mounted to two existing holes in work platforms through thefront I-beam of the mounting frame, and 600 lb of additional weight was added to the rear of the mounting frameto improve the force input to the flight test vehicle. Each hydraulic actuator was connected to the vehicle throughthe use of a 1/2” threaded rod connected to a PCB series 223 dynamic load cell mounted to an 8” squarealuminum plate, which was attached to the vehicle using Tridox F88 dental cement. An additional static load cellwas mounted on the actuator side of the threaded rod to monitor static load inputs. Although the actuators weredesigned to allow linear movement on the mounting frame, they were operated during the modal test with anadditional solenoid active to increase the rigidity of the actuator relative to the work platform. The hydraulic powersupplies and solenoids were controlled remotely at a location near the data acquisition system. Team model2240 valve drivers were used at this location to control the shakers based on Linear Variable DifferentialTransformer position feedback from each shaker and an input signal from the data acquisition system. Theshaker systems were located on two work platforms near the 4 th and 5th segments of the flight test vehicle’s firststage. Pre-test analysis showed that these locations were able to excite all modes of interest and that the highestchosen platform was an optimal location for exciting the 2nd through 4th bending modes. On each work platform,the shakers were oriented at 45° to the preferred direction of travel of the vehicle and 90° relative to each other.This was necessary in order to avoid flight test instrumentation cables and equipment attached to the vehiclewhile still maintaining an orthogonal shaker setup.

Figure 3. Hydraulic shaker setup during flight test

Figure 4. Electrodynamic shaker setup during Supervehicle modal test

Stack 1 modal test

The data acquisition system for the modal tests consisted of VXI Technology model VT1432B 24-bit digitizercards and a Hewlett Packard model 1434A source card in a single Agilent Technologies model E8403A 13-slotmainframe. These data acquisition and source cards were controlled through a Firewire connection to a dataacquisition computer running m+p International SmartOffice Analyzer software. The software allowed time historythroughput data to be recorded directly to disk while simultaneously computing, displaying and storing autopower,coherence, and frequency response functions during testing.

The Super Stack 5 modal test setup consisted of the top portion of the flight test vehicle bolted to a SuperSegment Assembly Stand and heavy weight Upper Stage Simulator transportation cart that rested on a concretefloor, as shown in Figure 5. Test instrumentation consisted of 70 accelerometers placed at 30 locations on thetest article in order to measure the first three bending mode pairs of the stack. Of these 30 locations, 10 werearranged in a triaxial configuration and 20 were biaxial. This test used two MB Dynamics model 250 electro-dynamic shakers for excitation of the vehicle, shown in Figure 4. Each shaker had a 2” stroke and was capable of

Figure 6. Test setup for Super Stack 1 modal test

providing 250 lb-peak force with supplemental forced air cooling. The shakers for this test were mounted towooden pallets that were lifted to a height of approximately 16 feet by JLG telehandlers. The shaker locationswere identified by the pre-test analysis as optimal locations within elevation constraints imposed by the project.The force input spectra to the structure remained flat with no significant dips, and additional triaxialaccelerometers mounted to the shakers showed that the telehandler modal frequencies were not at the targetmodal frequencies. Impact testing at the shaker input locations also showed that the frequency responsefunctions were consistent with those acquired using the shakers. These checks were performed both as a pre-test step on another structure and during the actual test to verify that the chosen shaker setup was acceptable.

The Super Stack 1 modal test setup consisted of the 5 th segment and interstage components in a buildupstand shown in Figure 6. Project constraints resulted in the Super Stack 1 segment being tested while resting on12 steel shims on an I-beam support structure on top of reinforced concrete pylons. Instrumentation was locatedat 45 locations, with 7 triaxial, 29 biaxial, and 9 uniaxial locations, totaling 88 accelerometers. The same 250 lb-peak electro-dynamic shakers from the Super Stack 5 test were used for this test, and they were located on awork platform near the middle of the stack. The shaker locations were influenced by project constraints, and pre-test analysis confirmed that the locations were satisfactory for exciting the modes of interest. The shakers werelag bolted to wooden fixtures to simplify alignment procedures, and these fixtures were bolted to the work platformin 2 locations near the front of the shaker, as shown in Figure 4.

TEST EXECUTION

The test execution for all three tests was similar, so only the flight test vehicle test will be described todemonstrate the process. More information on the test execution for the Super Stack 5 and Super Stack 1 modaltests are included in NASA technical memorandums 3,4,5 .

The flight test vehicle modal test data was acquired over the course of 3 test days. The vehicle was clear ofpersonnel during testing on these days. A summary of the high quality datasets that were acquired during thistime is given in Table 1.

Table 1. Flight Test Vehicle Modal Test Data SummaryType Quantity Force Levels Frequency Range Resolution Averages

Ambient Noise 2 n/a 0 - 12.5 Hz 0.0019 Hz 23

Random 3 50, 130, 200 lb-rms 0 - 6.0/12.5 Hz 0.0019 Hz 23

Sine Sweep 9 50, 100, 200 lb-peak Mode Dependent Mode Dependent 1

Free Decay 7 n/a Time Data 0.031 seconds n/a

Tap Test 11 50 - 800 lb-peak 0 - 200/400 Hz 0.125 Hz 8

The first datasets to be acquired were the two ambient noise tests. This test data was actually acquiredduring the test setup days, while people were still working inside and outside the vehicle. During one dataset thehigh bay doors were left open and wind loads were exciting the vehicle bending modes. This data was sampledat 16 Hz with a block size of 512 seconds (8.5 minutes) to achieve the desired resolution of 0.0019 Hz.Autopower data was processed with a Hanning window and a 50% overlap, and 23 averages were acquired for atotal test time of 1 hour and 42 minutes.

Acquisition of the 3 random datasets used similar parameters to the ambient noise tests, but frequencyresponse functions and multiple coherence functions were processed from the data in addition to the autopower.The frequency response functions were computed during acquisition using the H1 estimator. During all but the200 lb-rms random dataset, 4 shakers were operational and a bandwidth of 12.5 Hz was used for the force input.In order to focus more energy into the target modes, a bandwidth of 6.0 Hz was used for the 200 lb-rms randomtest, but the failure of an analog filter on one of the shaker source channels caused only 3 shakers to beoperational.

Selection of the resolution for the ambient noise and random datasets was a function of both the desired dataquality and the limited test time available. The desired resolution was based on a “rule of thumb” suggested byElliott and Richardson 6 that “5 or more samples above the 6-dB points, or the noise floor, of a resonance peak isconsidered acceptable for good curve fitting.” Based upon this rule, the predicted natural frequencies, and anassumed damping of 0.5%, a block size of 34 minutes was determined to be required for a good curve fit of the 1 st

bending modes. This can be seen in Figure 7, where the measured modal frequencies and damping values areplotted on a chart that was generated numerically (the predicted frequencies for these modes were very close tothe measured frequencies). Unfortunately, the 34 minute block would have resulted in unacceptable test times,so the 8.5 minute block was selected based upon the 2nd bending modes. The strategy for the 1 st bending modeswas to reprocess the data from the time histories so that the desired resolution could be achieved with feweraverages. Previous testing of Super Stacks 1 and 5 had indicated that 20 averages provided acceptable testresults, so the plan was to acquire 12 x 8.5 minute blocks, and process them to 23 averages with 50% overlap. Ifthe initial curve fit of the first random dataset was unacceptable, the time records could be increased in laterrandom tests.

Figure 7. Chart of desired block size versus natural frequency for various damping values

The sine sweep tests were performed with a single shaker and focused on exciting the 2nd and 3rd bendingmodes at various force levels as a linearity study. Because the shakers were mounted at 45° to the preferreddirection of each mode, a single shaker could excite the modes in both axes. For the 2nd bending modes, a lowerplatform shaker was used for excitation, and an upper platform shaker was used for the 3rd bending modes. Thiswas because the node locations of the 3rd bending modes were very close to the lower shaker platform. The 2 nd

bending modes had similar mode shape magnitudes at either platform, so the lower shaker was chosen for easyobservation from the data acquisition system. The sweep rate was chosen based on natural frequency anddamping ratios identified during a random test according to an equation listed in Modal Testing by Ewins7 .

S,,,a, < 310^f,,)^^,, ) 2 octaves/min (1)

The resulting sweep rates were 0.003 octaves per minute for the 2nd bending mode pair, and 0.01 octaves perminute for the 3rd bending mode pair. The force was controlled to a constant level during the sine sweeps by them+p SmartOffice software, but the lowest sweep rate that our version of the software would allow was 0.01octaves per minute. This was one of the consequences of lower than expected damping values. In order to workaround this situation, the plan was to sweep at the 0.01 octaves per minute rate with the knowledge that onlyabout 80% of the steady state response would be reached during the sweeps of the 2nd bending modes8 .

During the first sine sweep attempts on the 2nd bending modes, an issue occurred where the force level wentunstable after passing through the resonance and nearly doubled in force. The cause of this issue has not beenidentified. In order to work around this problem, the sine sweeps were stopped as soon as the force level wentunstable. The force instability for sweeps of these modes occurred after passing the 6 dB points, so the data wasstill considered to be acceptable. This instability was not a problem during the sweeps on the 3 rd bending modes,so the 3rd bending mode sweeps excited modes in both axes during a single sweep.

In order to investigate the damping linearity of the 1 st bending modes, several free decay tests wereperformed on the flight test vehicle. From the top work platforms next to the vehicle, several members of the testteam pushed on the vehicle in synchronization with the first bending mode period, approximately every 5seconds. The vehicle eventually responded with approximately 8” deflections at the top of the vehicle, which wasthe maximum deflection that could be safely reached while maintaining acceptable platform clearances. Whenthe maximum deflection was reached, the vehicle motion was allowed to decay naturally. The resulting timehistory data allowed the damping to be estimated using a logarithmic decrement approach. One more free decaytest was also performed in an attempt to target the 2nd bending mode in the Y-direction. The raw accelerationresponses from this test did not clearly indicate individual peaks, but band-pass filtering did allow this data to beused to estimate damping as well.

Figure 8. Frequency response functions and multiple ry

coherence functions from flight test vehicle random testFigure 9. Detailed view of split peaks in frequency

response functions

The final set of tests that were performed on the vehicle were tap tests with a 3 lb instrumented hammer. Thetap tests were intended to investigate local modes near three instrumentation locations for the Guidance,Navigation and Control system. These tap tests had a wider frequency bandwidth of excitation than the shakertests.

RESULTS AND DISCUSSION

Only the flight test vehicle results will are provided in this section. A companion paper 9 will provide a shortoverview of the measured mode shapes from all of the Ares I-X modal tests and their use in model calibration.More information on the test results for the Super Stack 5 and Super Stack 1 modal tests are included in NASAtechnical memorandums 3,4,5 .

Sample frequency response data from the 130 lb-rms random dataset is shown in Figure 8 below. The plotshows that the peaks corresponding to the 2nd through 4th bending mode pairs are clearly defined with minorcoherence drops at the peaks of the 2nd bending modes. The 1 st bending mode pair, however, had largecoherence drops where the peaks were split in half, as shown in Figure 9. Methods were sought to improve thecoherence near these peaks, such as the use of load cell signal conditioners that had a lower frequency responseand acquisition of unconditioned Integrated Electronics Piezo Electric (IEPE) signals from the load cells. Neitherof these methods resulted in improved low frequency coherence. The cause of these drops in coherence isunknown but thought to be related to the locations of the shakers and the ambient movement of the structure. Analternative explanation is related to the fact that the hermetic seals of the load cells may have been breached in aprevious modification, possibly changing the time constant of the circuit. The peaks in the autopower from theambient noise tests were clearly defined and did not show this split. Due to this difficulty in exciting the 1 st

bending mode, frequency and damping estimates were acquired using free-decay response measurements.

Another interesting phenomenon from the random datasets was the peaks that showed up in the load cellautopower data as shown in Figure 10. These peaks did not correspond to any of the peak response frequenciesidentified in the frequency response functions. The load cell spectra were otherwise flat across the bandwidth ofinterest.

Good reciprocity was exhibited for all cases in all of the random test datasets. A sample comparison betweenthe upper platform shakers for the 130 lb-rms dataset is shown in Figure 11.

Figure 10. Load cell autopower measurements from

Figure 11. Reciprocity in flight test vehicle random testflight test vehicle random test

data

A comparison of the same frequency response function measurements from the three random tests showsthat the target bending modes were linear for the tested excitation levels. A system level mode that includedmovement of the Mobile Launcher Platform did indicate some nonlinearity, but this was not a problem because itwas not a target mode for the test. The peak frequencies of the 2nd through 4th bending modes from theserandom tests are shown in Table 2. The table indicates that frequency shifts were all below 1 % from 50 to 200 lb-rms. The 2nd bending mode in the Z-direction had the greatest frequency shift at 0.82%, while the other bendingmodes did not shift by more than 0.34%.

Figure 12. Linearity of Y-direction frequency response Figure 13. Linearity of Z-direction frequency responsefunctions from flight test vehicle random tests functions from flight test vehicle random tests

Table 2. Peak Frequency Changes for Random Excitation

Random Random Random 50 vs. 200 lb-rmsMode 50 lb-rms Peak 130 lb-rms Peak 200 lb-rms Peak Percent

Frequency (Hz) Frequency (Hz) Frequency (Hz) Difference (%)

2nd bending Y-direction 1.0605 1.0586 1.0586 -0.18

2nd bending Z-direction 1.1953 1.1953 1.1855 -0.82

3rd bending Y-direction 3.4649 3.4590 3.4531 -0.34

3rd bending Z-direction 3.6856 3.6816 3.6738 -0.32

4th bending Y-direction 4.6133 4.6113 4.6074 -0.13

4th bending Z-direction 4.7910 4.7832 4.7832 -0.16

The sine sweep test data was also used to investigate modal frequency shifts for the 2nd and 3 rd modes athigher force levels than the random test. The test data was processed by taking a single Discrete FourierTransform of the entire time history for each channel and computing the frequency response functions by thesimple ratio of the acceleration spectra over the force spectrum 10. The resulting peak frequencies are listed inTable 3, which show frequency shifts less than 1.2% from 50 to 200 lb-peak. Again, the greatest frequency shiftwas observed for the 2nd bending mode in the Z-direction, while the other bending modes did not shift by morethan 0.18%.

Table 3. Peak Frequency Changes for Sine Sweep Excitation

Sweep Up Sweep Up Sweep Up 50 vs. 200 lb-pkMode 50 lb-pk Peak 100 lb-pk Peak 200 lb-pk Peak Percent

Frequency (Hz) Frequency (Hz) Frequency (Hz) Difference (%)

2nd bending Y-direction 1.0527 1.0527 1.0508 -0.18

2nd bending Z-direction 1.1836 1.1797 1.1699 -1.2

3 rd bending Y-direction 3.4600 3.4590 3.4590 -0.03

3 rd bending Z-direction 3.6777 3.6797 3.6729 -0.13

A sample time history from the 3rd free decay test of the 1 st Y-direction bending mode is shown in figure 14.The time history was low pass filtered with a 1.0 Hz Bessel filter in order to clean up the peaks, but this had littleeffect on the overall trends in the time history data. A plot of the natural logarithm of the peaks from the timehistory versus peak number indicated that the damping was linear over the tested amplitude range because theslope of the line is proportional to the damping ratio 11 . The resulting damping ratios from the free decay tests arelisted in Table 4. Additionally, the average damped natural frequency of the decay was easily estimated from thetime history data. This frequency was used along with the peak acceleration amplitudes to approximate thedisplacement of the top of the flight test vehicle during the test.

Figure 14. Y-direction free decay test data from the top

Figure 15. Linearity of damping from Y-direction freeof the flight test vehicle

decay test data

Table 4. Free Decay Test Results from the Flight Test Vehicle Modal Test

Maximum Maximum MaximumUnfiltered Unfiltered Fit Free Decay

Mode Decay # Amplitude Amplitude at Frequency Damping (%)During Start of Am(in)ude (Hz)

Push (in) Decay (in)

1 5.40 4.90 4.75 0.1776 0.81

1 st bending Y-direction 2 7.80 7.80 6.78 0.1764 0.88

3 8.28 8.28 5.98 0.1764 0.88

1 6.85 6.85 5.55 0.2149 0.45

1 st bending Z-direction 2 8.29 8.29 6.45 0.2142 0.43

3 7.82 7.82 6.97 0.2145 0.44

2nd bending Y-direction 1 0.0908 0.0793 0.0392 1.056 0.23

Final modal parameters from the modal test were estimated using different methods by several analysts.Most analysts used the H1 estimates for random excitation with industry standard parameter estimation routines.One more rigorous approach was used to try and improve the 1 st bending estimates using data from the 200 lb-rms random test. The frequency response functions for this curve fit were processed in a different manner thanthe processing method used during the data acquisition. The time histories from the accelerometers andIntegrated Electronics Piezo Electric (IEPE) load cell signals with biases removed were processed by taking cyclicaverages, then applying a Hanning window and processing the blocks with a 75% overlap. The Hv estimator wasused to compute the frequency response functions. The curve fit used the PolyMAX algorithm 12 , and only the Yand Z-axis measurements were included during the pole estimation process. The results from this method andseveral others are included in Table 5, which summarizes the final target modal parameter estimates for the AresI-X flight test vehicle on the Mobile Launcher Platform. Because of the issues with the 1 st bending modes in therandom datasets, the frequency and damping estimates for these modes were taken from the free decay data.Free decay data was also used in addition to the random data to estimate the damping of the 2nd bending modes.Overall, the data compiled from several different analysts, parameter estimation routines, and datasets is veryconsistent. A comparison of these average measured frequencies to the predicted frequencies is listed in Table6. The comparison shows that the majority of the measured frequencies were within 5% of predictions, with oneexception at 6.4% for the 3rd bending mode in the Y-direction. More information about the use of the measureddata in the post-test analysis is included in a companion paper 9 .

Table 5. Summary of Modal Parameter Estimates from Ares I-X Flight Test Vehicle Modal TestAverage STD Average STD Min Max

Mode (H)(H)

Da^)p Damp(%) Damp(%) (a/0)p Data Source

1 st Bending Y 0.178 0.002 0.848 0.038 0.81 0.88only free-decay

direction data1 st Bending Z 0.213 0.002 0.433 0.017 0.41 0.45

only free-decaydirection data

2nd Bending Y 1.06 0.001 0.290 0.034 0.23 0.31 free-decay anddirection random

2nd Bending Z 1.19 0.004 0.372 0.081 0.25 0.46free-decay and

direction random3rd Bending Y 3.46 0.003 0.483 0.036 0.43 0.51 random datadirection3rd Bending Z 3.65 0.002 0.388 0.015 0.37 0.40 random datadirection4th Bending Y 4.61 0.001 0.175 0.006 0.17 0.18 random datadirection4th Bending Z 4.78 0.003 0.243 0.010 0.23 0.25 random datadirection

Table 6. Comparison of Predicted and Measured Frequencies

Mode PredictedFreq (Hz)1

MeasuredFreq (Hz)

PercentDifference

(%)1 st Bending Y direction 0.176 0.178 1.1

1 st Bending Z direction 0.216 0.213 -1.4

2nd Bending Y direction 1.02 1.06 3.7

2nd Bending Z direction 1.17 1.19 2.0

3rd Bending Y direction 3.25 3.46 6.4

3rd Bending Z direction 3.50 3.65 4.2

4th Bending Y direction 4.78 4.61 -3.6

4th Bending Z direction 4.84 4.78 -1.2

CONCLUSIONS

The target modal parameters for the Ares I-X flight test vehicle were obtained using both free decay andrandom datasets. The 1 st bending mode pair was difficult to curve fit from the random data due to inadequateresponse measurements at lower frequencies that split the peaks of the frequency response functions, so thenatural frequencies and damping ratios were determined from free decay data. The 2nd through 4th bendingmodes were well defined in the random data. Additionally, sine sweep tests were performed on the 2nd and 3 rd

bending modes to investigate frequency nonlinearities. Results from multiple levels of random and sine sweeptesting indicated linear frequency behavior of the modes, with maximum frequency shifts of 1.2% for quadruplethe force levels. Overall, the modal test successfully identified the modal parameters for all of the targetedbending modes of the Ares I-X flight test vehicle, and the measured modal frequencies were in good agreementwith pre-test predictions.

REFERENCES

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2. Kennedy Space Center Media Gallery, http://mediaarchive.ksc.nasa.gov/search.cfm?cat=166, October 2009.

3. Buehrle, R. D.; Templeton, J. D.; Bartolotta, P. A.; Reaves, M. C.; Horta, L. G.; Parks, R. A.; Lazor, D. R.;Gaspar, J. L.: Ares I-X Flight Test Vehicle: Stack 5 Modal Test NASA/TM-2010-216183, January 2010.

4. Buehrle, R. D.; Templeton, J. D.; Bartolotta, P. A.; Reaves, M. C.; Horta, L. G.; Parks, R. A.; Lazor, D. R.;Gaspar, J. L.: Ares I-X Flight Test Vehicle: Stack 1 Modal Test, NASA/TM-2010-216210, March 2010.

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9. Horta, L. H., et. al., “Finite Element Model Calibration Approach for Ares I-X,” Proceedings of IMAC XXVIII,February 2010.

10. Orlando, S., Peeters, B., and Coppotelli, G., “Improved FRF Estimators for MIMO Sine Sweep Data,”Proceedings of the ISMA 2008 International Conference on Noise and Vibration Engineering, Leuven, Belgium,September 2008.

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12. Peeters, B, et. al., “The PolyMAX Frequency Domain Method: A New Standard for Modal ParameterEstimation?” Journal of Shock and Vibration, Vol. 11, pp. 395 – 409, 2004.